The present invention relates to a medical X-ray imaging apparatus comprising a frame part, an X-ray source for producing X-rays, radiation receiving means for detecting radiation transmitted through an object, support means for supporting the X-ray source and the receiving means in positions on opposite sides of the object, the said support means being connected to the frame part so as to rotate around the axis of rotation, and actuator means for rotating the support means around the said axis of rotation, the apparatus making possible both panoramic imaging and CT imaging, as selected by the operator.
In odontological panoramic X-ray imaging, X-rays emitted from the X-ray source during imaging are guided through the dental arch to the receiving means for producing an image and the support means are rotated to produce an image of essentially the whole dental arch. In panoramic imaging, the aim is to produce images of the teeth in projections that are as perpendicular to the dental arch as possible in order not to produce images of the teeth covering one another. Since the dental arch deviates from a circular shape, the axis of rotation of the support means must be moved during imaging to make perpendicular imaging possible. This type of a panoramic imaging apparatus is known, for example, from the publication U.S. Pat. No. 6,744,847.
In CT imaging, several projection images are taken of the object from different directions, which are then reconstructed to form the desired layer images. A prior art CT imaging apparatus and method are described in the publication U.S. Pat. No. 6,434,214.
A panoramic X-ray imaging apparatus is typically only intended for panoramic imaging and a CT imaging apparatus only for CT imaging and thus an aim in field has been to combine the apparatuses to save on expenses and space. One problem in combining the apparatuses is that the radiation receiving means are located at a standard distance from the focus of the X-ray source, the said distance being a compromise between the different distances required by these two different imaging methods, which means that the ratio of enlargement is not optimal for each imaging method. The ratio of enlargement is determined as the ratio (SID/SOD) of the distance (SID) between the focus of the X-ray source and the image plane (active surface of the detector) to the distance (SOD) between the focus of the X-ray source and the object being imaged (image layer).